This invention relates to a surface acoustic wave device for filtering electrical signals, and in particular, to a surface acoustic wave device having a tilted transducer and a normal transducer.
Surface acoustic wave devices have been widely used as electronic filters. Various types of filters using surface acoustic wave devices (hereinafter called SAW filters) have already been proposed for diverse applications. Among them is a SAW filter which has a tilted transducer and a normal transducer as input and output transducers, proposed by the present inventors (see the preliminary report for the National Convention of the Electrocommunication Society in 1981, page 1-68, published by the Japan Electrocommunication Society). This SAW filter has been designed for use as a VSB (vestigial sideband) filter in television broadcasting transmitters where both a large relative bandwidth and a frequency-amplitude (transfer) characteristic of low "shape factor" are required. (Shape factor is defined as the ratio of the -30 dB bandwidth to the -3 dB bandwidth, 0 dB being measured at the center of the pass band.) The SAW filter with one normal and one tilted transducer is more compact than filters in which both the input and output transducers are tilted and coupled by a multistrip coupler. However, the conventional SAW filter using a combination of a tilted transducer and normal transducer has several deficiencies. First, it has a "guaranteed attenuation" (the difference in attenuation of signals between the pass band and the rejection band) of only 30 dB, which is insufficient. Second, it has an undesirably large amplitude ripple of .+-.0.25 dB in the pass band.